We humans are subject to hyperbolicdiscounting, not taking full account of long distant future costs for current pleasures, sometimes those running the public health system really do know more than us, there are externalities associated with these behaviours (late night drunks, passive smoking and the visual pollution of someone 300 lbs overweight perhaps).
One of the biggest problems in personal finance is a phenomenon that behavioral economists call hyperbolic temporal discounting, which is basically a fancy term for our tendency to sacrifice the future to live for today.